The invention relates to an objective lens for a microscope and, in particular, to such an objective lens which maintains a good correction of aberrations due to large variations in the thickness of cover glasses.
An objective lens for a microscope is generally designed to achieve a good correction of aberrations when a cover glass having an optical path length d.sub.0 is used. However, cover glasses are not always manufactured to a given thickness, and, in the past, this has prevented a viewer from performing a microscopic examination with a cover glass having a different optical path length d.sub.0 from observing a sharp image when an objective lens is used which is well-designed to achieve a good correction of aberrations for a cover glass of optical path length d.sub.0.
For microscope objective lenses having numerical apertures (hereafter abbreviated as NA) which are equal to or greater than 0.3 to 0.4, changes occurring in the aberrations which result from a variation in the thickness of a cover glass interposed between a front lens and an object being observed can no longer be neglected. In particular, for NA values which approach 0.6, changes in the aberrations increase in an accelerated manner with an increase in the numerical value, thus distorting the image of the object to be viewed. This can be clearly appreciated from FIG. 3 which graphically indicates the behaviour of the spherical aberration for so-called d-line of NA 0.5 objective lens with and without 1 mm thick cover glass, the lens being corrected for aberrations when a 1 mm thick cover glass is used.
Japanese Laid-Open Patent Application No. 142,508/1981 proposes a technique in which a suitable lens spacing is chosen and telescoped by a cam which is mechanically coupled with a correcting ring to provide correction against changes in the aberrations which result from the varying thicknesses of cover glasses. However, with this proposed technique, a good correction of aberrations is precluded for a large variation in the thickness of the cover glass or for increased numerical aperture values. Thus a limited range of available correction results, and the aberrations are appreciably exacerbated towards the limits of the range. In addition, the technique allows a change in the spherical aberration to be corrected, but fails to correct for a change in the chromatic aberration. As a consequence, with an apochromatic objective lens which has its chromatic aberration corrected down to so-called g-line, the correction of aberration is possible with a variation in the thickness of the cover glass only up to the order of 0.2 mm.